This invention relates generally to a switching circuit and, more particularly, to a switching circuit which initiates and terminates current flow to an AC connected load at the time in the AC cycle when input voltage is at or near zero. That is, energization and de-energization of the load occurs in substantial synchronism with the axis crossing of the voltage waveform. Also, power is furnished only in complete, integral AC cycles.
It is known that activation and deactivation of an AC load at a time of substantial instantaneous line voltage can create annoying, as well as harmful, side effects in the power source and connected loads. Also, nearby sound, communication, data processing and video circuits can be adversely affected by transients introduced by switching. Additionally, interruption of power in mid-cycle may have harmful effects by exposing transformers and motors to rectified power, and magnetic components may retain an undesired residual magnetism.
U.S. Pat. No. 3,727,080 by Hanchett and U.S. Pat. No. 3,745,378 by Pritchett are examples of prior art circuits which provide so-called zero voltage switching with an integral power cycle. The latter circuit is quite complex, and the former patent appears to use substantial gate power during the first half cycle which may shorten the operational life of the elements. Additionally, Hanchett requires an externally supplied voltage signal to arm his circuit for load operation.